Semiconductor nanorods are often considered as quasi-one-dimensional (1D) systems. They manifest linearly polarized emission, reduced lasing threshold and improved charge transport as compared with their counterpart spherical quantum dots. Present investigations of colloidal semiconductor nanorods are mainly based on the system of cadmium chalcogenides due to their facile synthetic accessibility.
However, it is still a challenge to fabricate quasi-1D zinc chalcogenide nanocrystals with controlled aspect ratios. Development of such nanocrystals are of high importance, as the availability of cadmium-free (or “green”) zinc chalcogenide nanorods, produced through this strategy, provides a desirable platform for eco-friendly components for use in, e.g., optics, photocatalysis, electronic devices and bio-labeling.
Selective monomer attachment and coalescence of particles are among the most important growth mechanisms of quasi-1D colloidal nanocrystals. On one hand, the growth rates of different facets of a nanocrystal are determined either by the internal growth behavior of these facets or the binding energy between the facet and the organic surfactant molecule. So monomers attach much faster to the high-energy facet than the others, leading to the formation of elongated nanostructures. On the other hand, nanorods can be produced by the co-alignment and coalescence from isolated individual building blocks via an oriented attachment process.
Compared with cadmium chalcogenide nanorods (CdS, CdSe and CdTe), which have been intensively studied, zinc chalcogenide nanorods (ZnS, ZnSe and ZnTe) are hardly investigated due to the difficulty in their synthesis, which is related to the stability of different crystal structures, not necessarily being compatible with anisotropic growth. The seeded growth approach in the presence of phosphonic acid as the surfactant in trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO) solvents produces CdSe/CdS quantum rods with excellent size distribution and controlled aspect ratio [1-3]. However, attempts to use this approach to synthesize quasi-1D zinc chalcogenide nanorods have been unsuccessful, resulting in either polycrystalline or polypod-shaped zinc chalcogenide nanoparticles with poor size distributions [4-9].
It has been demonstrated that alkylamines, rather than TOP/TOPO, which act as both surfactant ligands and solvents, are more suitable for synthesizing zinc chalcogenide nanorods [10-14] due to the better compatibility between alkylamine ligands and zinc ions; yet the control of the diameter of the nanorods has not been achieved. In these cases, the thermal decomposition of organic precursors produces tiny ZnSe and ZnS spherical quantum dots. Instead of a dopting monomer attachment growth mechanism, the produced individual quantum dots co-align and coalesce into elongated nanowires. However, the diameters of the produced nanowires are mainly below 3 nm [11-12, 15-20], suggesting that the limitation of width control of zinc chalcogenide nanorods imposed by alkylamines is hard to be circumvented.
Recently, Achraya et al. [21] reported synthesizing metal Mn2+ doped ZnSe 1D nanorods via fragmentation. The ion doped 1D nanorods growth involved utilizing (crude) sample of nanowire precursors and swift injection to hot solutions.